The present invention is directed to fluid reactor systems and techniques. More particularly, but not exclusively, the present invention is directed to the fabrication and use of microchannel chemical reactors with temperature control for performing equilibrium limited reactions such as chemically reversible reactions or multiple competing reactions.
For some reactions, for example certain single reactions that are chemically irreversible or endothermic, maximizing the reaction temperature is often desired because both kinetics and conversion increase with increasing temperature. However, for many reactions, trade-offs exist between kinetics, equilibrium, and reaction selectivities. For example, reversible exothermic chemical reactions generally exhibit improved reaction kinetics but lower equilibrium conversion with increasing temperature. Lowering the reaction temperature favors higher conversion but typically requires more catalyst and a larger reactor. Accordingly, more efficient utilization of catalyst and reactor resources for a desired conversion likely requires a non-uniform temperature trajectory for the reactants as they progress through the reaction process. For example, it has been found that for a single reversible exothermic reaction, such as the water-gas-shift (WGS) reaction, a theoretical optimal temperature trajectory would start at a high temperature to take advantage of fast kinetics and proceed in monotonically decreasing fashion to lower temperatures to improve conversion. More complex optimal temperature trajectories are possible with reaction sequences or competing reactions.
There are also reasons related to energy efficiency and exergetic efficiency to control the temperature trajectory of chemical reactions. For both endothermic and exothermic chemical reactions, greater thermodynamic reversibility, and therefore greater system efficiency can theoretically be achieved with reaction temperature control.
One conventional method for controlling the temperature trajectory for exothermic reactants as they flow through a reactor system is to employ a sequence of separate adiabatic reactors and heat exchangers [Levenspiel, O., Chemical Reaction Engineering, 2nd Ed., John Wiley & Sons, Inc, New York, 1972, pp.509-516]. In this approach, the outlet stream from one adiabatic reactor is cooled in a heat exchanger prior to being fed to the next successive reactor. However, within each reactor, the temperature increases down the length due to the heat of reaction. Consequently, a plot of the temperature through the series of reactors is saw-toothed rather than monotonically decreasing.
A sequence of two water-gas-shift reactors with an intervening heat exchanger is the typical approach for fuel processors being developed to produce H2 from liquid fuels for fuel cell power applications. [Petterson, L. J. and R. Westerholm, Int. J. Hydrogen Energy, 26, (2001), 243]. In this application, the outlet from a fuel reformer is fed to a pair of shift reactors in series. The reformate is first reacted at about 400° C. in a high temperature shift (HTS) reactor, with the outlet stream of the HTS reactor cooled to around 250° C. prior to introduction in a second shift reactor. Overall conversion of the CO to CO2 is typically about 90%.
Macroscale packed-bed reactors have also been employed to improve the temperature trajectory for reversible exothermic reactions. One example is the Tennessee Valley Authority (TVA) ammonia synthesis reactor, which was simulated by Baddour et al. [Baddour, R. F., P. L. Brian, B. A. Logeais, and J. P. Eymery, Chem. Eng. Sci., 20, (1965), 281]. The TVA ammonia synthesis reactor consists of an array of 5 cm outer-diameter tubes penetrating through a packed catalyst bed. However, in this reactor temperature differences between the hot and cold stream at a given cross-section are on the order of 200° C., implying large thermal gradients across the bed and/or high heat transfer resistance.
Accordingly there exists a need for improvements in the art of reactor design to provide reactors with improved temperature control and that enable better and more precise control of reaction temperatures.